Grand Targhee

000
FXUS65 KRIW 010006
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
606 PM MDT Sat Oct 31 2026

.KEY MESSAGES...

- Isolated showers continue across the west tonight with
  increasing chances early Sunday morning.

- A potent, but fairly quick-moving, weather system crosses the
  area from the west Sunday into Monday. Widespread rain 
  showers and high elevation snow is expected, with all 
  locations having a chance (at least 50%) to see moisture. 

- A cold front brings gusty north winds Sunday night through
  Monday. The strongest wind, over 50 mph, is expected for the
  Bighorn Basin and Johnson County, though gusty winds occur for
  much of central and southern Wyoming.

&&

.UPDATE...
Issued at 1237 PM MDT Sat Oct 31 2026

The main change with this update is the addition of a Winter 
Weather Advisory for the western Wind River Mountains. The 
heaviest snow is expected to occur between about 11am and 11pm 
MST Sunday, though wind may linger impacts slightly longer. The 
highest totals are most likely going to be confined to 
elevations about 9500 feet and higher. Therefore, locations like
South Pass see lesser impacts, with only a couple inches 
possible at the highest point. Consideration for an advisory was
made for around the Tetons. However, snow amounts appear less 
here, with notable impact locations like Teton and Togwotee 
Passes not reaching criteria. Still, Togwotee Pass should see a 
few inches of snow Sunday afternoon and into Sunday night, so 
minor impacts are possible. The other location of interest is 
the Bighorn Mountain snow and surrounding lower elevation wind. 
The timing here is later, and with some lacking confidence, will
forgo any highlights for now.

&&

.DISCUSSION...
Issued at 120 AM MDT Sat Oct 31 2026

Well, the day has arrived. Some people think it is the best holiday 
of the year, and in my opinion it is one of the best. A chance to be 
someone or something else, and of course, copious amounts of candy. 
The question is whether or not the weather will be spooky or if the 
weather will give us a trick or a treat on this last day of October 
and beyond. Well, you will find out in the discussion, that is why 
you are here after all. 

We still have the two weather systems that will impact the area over 
the next three days or so. The first is now moving northward through 
Utah and Colorado and will cross the area today, lifting northward 
into Montana by later this afternoon. This is the weaker of the two 
systems. Reasoning remains fairly similar to yesterday. It is that 
the most numerous showers will occur over western Wyoming (as high 
as a 4 out of 5 chance) with tapering chances further east. A shower 
can't be ruled out anywhere, but the smallest chance would be in 
Johnson and Natrona Counties where the chance is only 1 out of 10. 
The most numerous showers would occur through early afternoon, 
tapering off later in the afternoon with a lull expected to last 
most of the night. So, it looks like the weather has given us a 
treat when it comes to trick or treating earlier in the evening and 
for any adult gatherings later at night. The main chance of a shower 
this evening would again be in western Wyoming, but the chance 
is less than 1 out of 4, at least through midnight. As for 
amounts, the highest amounts would be focused in areas prone to 
southwest upslope flow, like the western Wind River Range. The 
chance of two tenths or greater would be confined to areas West 
of the Divide. Snow levels will remain above 8000 feet through 
this time as well, so impacts to travel would be minimal.

The main event still looks to be Sunday into Monday. The main 
players continue to be a long wave trough that will cross Wyoming 
Sunday night into Monday with associated upper level low that will 
pass into Colorado and then move northeast, inducing some lee 
cyclogenesis over Colorado. This will also phase with a stream of 
Pacific moisture as it moves into the Rockies. It still looks like 
the deepest moisture and heaviest precipitation will remain south of 
the area over Utah, Colorado and the desert Southwest, but most 
areas should see some precipitation from it. And, this will be a 
tale of two different upslope/downslope patterns. The steadiest
and heaviest precipitation on Sunday into Sunday evening will 
occur in western Wyoming and ares favored by west to 
northwesterly upslope, mainly to Tetons, Salt and Wyoming
Ranges, and the western Wind River Range. Then from later 
Sunday night into Monday, as the low passes to the east, the 
heaviest will shift East of the Divide to places favored by 
northerly upslope as moisture wraps around the developing low. 
This favors places like from south of Riverton to Lander and 
eastward to Casper, the southern Bighorn Basin from Thermopolis 
to south of Worland, as well as the Cody Foothills and the Bighorn
Range. With the north flow, places like the eastern Bighorn 
Basin, northern Wind River Basin, and much of the Green River 
Basin would see the least with downsloping flow.

Then there are two more questions, how much total precipitation and 
potential snow and how much could fall. We will start with 
precipitation. Most areas, except the southern Green River 
Basin, have at least a 1 in 2 chance of a quarter of an inch or
QPF between 6 am Sunday and 6 am Tuesday. The aforementioned 
favored areas that were mentioned above are the areas what have 
at least a 1 in 2 chance of over three quarters of an inch of 
QPF in that same timeframe. The greater than 1 in 2 chance of 
inch or more is largely restricted to the highest elevations 
though. Precipitation should taper off by sunset Monday, with 
all areas dry by Tuesday morning.

Now for the more juicy or should I say, cooler question - what 
about snow? The air with this system is of Pacific origin, and 
as a result not the coldest in the world. The coolest 700 
millibar temperatures look to drop to is minus 4, which would 
keep snow levels at roughly 6500 feet or higher. So, most 
population centers and roads outside of the mountain passes 
would see mainly rain or only wet snow that would not cause many
problems. Most locations above 7500 feet should see some 
accumulation though. As for advisory amounts of snow (6 to 12 
inches of snow in the mountains), the chance of this is largely 
above 8500 feet. However, this does include Teton and Togwotee 
Passes in western Wyoming as well as Granite and Powder River 
Passes in the Bighorn Range. We will likely (a greater than 3 
out of 5 chance) have some advisories in the mountain ranges. 
However, with main impact Sunday afternoon to Monday morning, we
will hold off for now and would issue them this afternoon or at
this time tomorrow night. As for the chance of greater than a 
foot, the best chance would be in the Bighorn Range where there 
would be a longer period of upslope and wrap around moisture. 
There are decent areas of a greater than 1 in 2 chance of 
greater than a foot of snow but these are mainly above 10500 
feet and above pass level. So, we favor advisories at this time.


And there is one more concern for Monday. As the low deepens over 
the plains, the pressure gradient will tighten and bring some
gusty to strong wind. Some of the latest guidance is showing at
least 35 knots of northerly flow at 700 millibar flow across 
the eastern two-thirds of the area. There are even some 50 knot
barbs across portions of Johnson County on Monday afternoon. 
The NBM ensemble shows a greater than 1 in 2 chance wind gusts 
over 55 mph for this timeframe so we may need a high wind watch 
for Monday as well.

Quieter weather looks to move in for Tuesday and Wednesday with 
gradually warming temperatures. A weak system may bring some showers 
on Thursday but it does not look like a big deal at this time. 
Temperatures should average near to above normal from Tuesday to the 
end of the workweek on Friday.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 331 PM MDT Sat Oct 31 2026

The next weather system is sweeping in from the southwest, and
is already bringing isolated showers to sites west of the divide
this afternoon. Timing still looks on track for the more
widespread event to start at sites west of the divide around
06Z, with mainly rain, although some mixed in snow may occur
around sunrise at the western sites. Clouds will remain MVFR to
IFR through the afternoon, with cigs likely at or below 1000 ft.
Low clouds and precip spread east of the divide after 12Z,
reaching KLND/KRIW midmorning and remaining sites around 18Z.
Remaining sites will also have cigs down to around 1000 feet
through the day. KRKS may start to see some breaks around 00Z,
but low cigs will likely continue into the early overnight 
hours.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 8 AM Sunday to 5 AM MDT Monday 
for WYZ014.

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...Straub